The present invention relates to a reactor riser of a fluidized bed catalytic cracking plant. Such a reactor riser has an axial passageway extending between an inlet end for receiving hydrocarbonaceous feed and catalyst particles and an outlet end for discharging effluent and catalyst particles.
In addition to the reactor riser, a fluid catalytic cracking plant may include a reactor vessel into which the outlet end of the reactor riser debouches, and a regenerator vessel. During normal operation, regenerated catalyst particles and hydrocarbonaceous feed are supplied to the inlet end of the reactor riser. In the riser the feed is vaporized, and a dispersion of catalyst particles in a gaseous mixture of feed is formed. In the reactor riser catalytic cracking of the feed takes place, and a gaseous mixture of feed and product is obtained. Because the composition of the gaseous mixture of feed and products changes along the riser reactor, this mixture will be referred to as xe2x80x98effluentxe2x80x99. The dispersion of catalyst particles in the gaseous effluent leaves the reactor riser at a temperature of between 500 and 540xc2x0 C. or higher. The dispersion is passed into a separator system in the reactor vessel where gaseous effluent is separated from catalyst particles. The gaseous effluent is removed from the upper end of the reactor vessel, and the catalyst particles are discharged to the lower part of the reactor vessel where they are stripped. Stripped catalyst particles are passed to the regenerator vessel where coke deposited on the particles during cracking is burnt-off at a high temperature to obtain combustion products and regenerated catalyst. The combustion products are removed from the upper end of the regenerator vessel and regenerated catalyst is reused. Normally the reactor riser is vertical.
In the reactor riser, the average linear gas velocity is in the range of from 8 to 30 m/s and the average linear velocity of the catalyst particles is up to 25 m/s. The catalyst particles will move substantially con-currently with the gaseous reaction mixture, and it is preferred that there is little slip between gas and particles.
As the cracking reaction takes place on the catalyst particles, good contacting between the gaseous effluent and the catalyst particles is essential for a sufficient degree of conversion and selectivity to the desired products, like for example gasoline.
To improve the contacting between the catalyst particles and the gaseous effluent, it is proposed in U.S. Pat. No. 3,353,925 to provide the reactor riser with a plurality of contacting devices arranged axially spaced apart in the axial passageway of the reactor riser. The known contacting devices comprise an annular mixing element, wherein the central plane of the element is arranged perpendicular to the central longitudinal axis. Consequently at a contacting device, the passage has a diameter that is smaller than the diameter of the axial passageway. When using such a mixing element that does not penetrate too much into the passage, an improved mixing of gas and solids and a decrease in the pressure drop is observed. The decrease in pressure drop results from the fact that the up-flowing gas can carry the solids in a more efficient manner because the gas and solids are better mixed.
It would be desirable to improve the gas-solids mixing even more. This can be achieved by increasing the penetration into the passageway of the mixing device as disclosed in the ""925 patent. However, by increasing the penetration depth of the mixing device into the passageway above a certain value, the pressure drop will increase instead of decrease, compared to when no internals are present. This increase in pressure drop will be too high for practical applications, for example erosion of the mixing element will likely occur.
Applicant now considers a contacting device which has a relatively large penetration depth, while the pressure drop over the contacting device is maintained at an acceptable level. The pressure drop is even lower when compared to the pressure drop over the contacting devices as described in the earlier referred to ""925 patent.
The apparatus according to the present invention comprises a fluidized-bed catalytic cracking plant riser reactor having an axial passageway extending between said inlet end for receiving hydrocarbonaceous feed and catalyst particles and wherein said outlet end discharges effluent and catalyst particles, which reactor riser is provided with a plurality of contacting devices arranged axially spaced apart in said axial passageway, wherein each said contacting device comprises a mixing element having the shape of a segment of arc, wherein the mixing element of each said contacting device is present in a plane perpendicular to the central longitudinal axis of the passageway and wherein the mixing element of a contacting device is arranged staggered with respect to the mixing element of an adjacent contacting device.
An advantage of the present apparatus is that the mixing of solids and gas is enhanced, resulting in higher feed conversions and product selectivity""s, while the pressure drop remains low. The non-radial symmetry of the contacting device is the reason for this enhanced mixing, and can be considered as a kind of xe2x80x9cstatic mixerxe2x80x9d. A further advantage of the mixing element of the present invention is that it can easily be built into the axial passageway of an existing reactor riser.
Because the mixing elements are arranged in a single plane perpendicular to the longitudinal axis of the passageway the relatively slow moving solids along the wall of the passageway are forced into the central region of the passageway. In the central region of the passageway higher gas velocities are present. By forcing the solids into this central region of the passageway a more uniform contacting between gas and solids is achieved. Mixing devices which impart a swirl movement, like elements having a spiral or helical design, and which will not be arranged in a single plane perpendicular to the axis of the passageway, will not achieve the desired effect. This is because, due to the swirl movement and the resulting centrifugal forces, solids will be forced to the wall of the passageway, resulting in a less effective contacting between gas and solids.
Because the mixing elements of the adjacent contacting devices are arranged staggered with respect to each other, all or most of the solids moving along the inner wall will be more effectively mixed with the co-currently moving hydrocarbon gas during its passage in the reactor riser.